The present invention relates to a multi-plate injection molding device or tool having multiple cavities and a hot runner distribution block for injection molding of multilayer preforms, in particular forms made of the thermoplastic resin PET, which is suitable for blow-stretching, as well as to a hot runner block for such a molding device and a valve for control of the injection of multiple thermoplastic resins.
Molding devices for injection molding multiple multilayer forms made of PET-like materials are well known and are extensively used in the production of preforms, such as are required in large numbers by the beverage industry for so-called PET bottles. These molding devices must meet unusually high standards because in processing PET and its copolymers the specific features of the material must be taken into careful consideration. For example, the processing temperature of PET has proven to be a critical factor. If the temperature is too low, then crystalline zones occur, which may result in cloudiness and brittleness of the material; if, on the other hand, the temperature is too high, then a high acetaldehyde content occurs and the PET begins to disintegrate, which, especially in the beverage and food industry, leads to an undesired impairment in the taste of the product. Also, PET reacts extremely sensitively to pressure fluctuations and shearing forces, which can also lead to local crystallization effects. Therefore, there must be kept in mind not only the laminate flow reaction of the PET mass and the injection rate, but also the regular temperature and pressure distribution within the PET material. These and other parameters affect in particular the regularity and homogeneity of the layer thicknesses as well as the density distribution within the manufactured preforms.
A device or tool with which multilayer preforms can be manufactured is very complex and is, for example, described in EP-307,058. This patent describes a hot-runner block for eight form cavities and five different synthetic or polymer masses. The individual synthetic masses are caused to fill separate press-out cylinders and are guided to the individual nozzles arranged in the hot runner block over a single, mutual mouth piece. Unfortunately, this construction has proven to be extremely complicated to maintain; in particular because of the T-and Y-splitters used, it is not easily adjustable and is not suitable for more than eight form cavities. In particular, the temperature conditions in the individual channels cannot be satisfactorily balanced because, among other things, they lie on different levels in the hot runner block and are therefore exposed to the temperature gradients inherent in the hot runner block. The small diameters of the individual channels lead to friction manifestations in PET material which undesirably alter the material, and also, this device does not permit a feed pressure of more than 345 bar, because otherwise it begins to leak.
In U.S. Pat. No. 4,710,118 there is, for example, described apparatus which is suitable for processing PET. The described construction with four form cavities is designed to work with three different materials, new PET, recycled PET and another thermoplastic material. The plasticized materials are each fed into a dosing unit by a squeezing cylinder and are pressed through a distribution valve to fill the cavities. Unfortunately, this construction is also not suitable for processing PET form masses, in particular because the channels through which the plasticized material is conveyed to the nozzles have different lengths, thereby causing the filling density in the separate form cavities to differ, which is undesirable. Aside from this, the temperature regulation of the conveyed material is extremely complex and insufficient, and the shearing forces acting on the material in the distribution valve produce undesired crystallization effects.
In the injection molding device as described in, for example, PCT Publication WO 91/16188, the above noted drawbacks do not occur. In this device, special attention has been paid to the length of the conveyor channels, thereby resulting in a regular temperature distribution in the especially constructed distribution blocks. Unfortunately, however, it is not possible with the disclosed structure to avoid an undesirable mixing of the form masses, which is not acceptable, in particular when manufacturing preforms for the beverage industry. Further, the use of numerous distribution blocks causes problems in the fine adjustment and thereby in the maintenance of the equipment. This system is therefore totally unsuitable for the manufacture of preforms comprising differently large proportions of different materials.